class_docstr: Add the `DefaultTokens.MASK_BEFORE` placeholder between
  the prompt and the response in an LM finetuning exemple.
  This is necessary to enable the 'zero-out prompt loss' mechanism.
docstr: Options for mask_before placeholders insertion
docstr_tok: ["Options","for","mask_before","placeholders","insertion"]
code: def add_options(cls, parser):
      """Options for mask_before placeholders insertion"""
  
      group = parser.add_argument_group(
          "Transform/InsertMaskBeforePlaceholdersTransform"
      )
      group.add(
          "--response_pattern",
          "-response_pattern",
          type=str,
          help="Response patten to locate the end of the prompt",
          default="Response : ｟newline｠",
      )
  
file_path: OpenNMT-py/onmt/transforms/insert_mask_before_placeholder.py
imports: from onmt.constants import DefaultTokens
  from onmt.transforms import register_transform
  from onmt.utils.logging import logger
  from .transform import Transform
label_desc: Use image node_id 2 for calling the InsertMaskBeforePlaceholdersTransform obj's underlying member method code with example usage: obj.add_options(cls, parser) without return types